The present disclosure relates to a light guiding member that guides light from a light source and linearly emits the light, an image reading device that reads an image of a document illuminated with the light from the light guiding member, and an image forming apparatus that forms an image on a sheet based on the image read by the image reading device.
A scanner device is known which includes an illuminating unit that illuminates a document by using a rod-shaped light guiding member that guides light from a light-emitting diode (LED). The periphery of the light guiding member is provided with a reflective member on one side. The reflective member reflects light so as to guide it in a direction orthogonal to a direction in which the light guiding member extends. The document is illuminated with the reflected light extending linearly in a main scanning direction. This illuminating unit, which has the LED on only one side adjacent to an input surface at an end of the light guiding member, is advantageous in that it can reduce the number of LEDs. This illuminating unit is also advantageous in that it can reduce a loss of LED light, because light guided into the light guiding member propagates inside the light guiding member while being totally reflected by the inner surface of the light guiding member in accordance with Snell's law.
In such an illuminating unit, characteristics of light distribution in a sub-scanning direction vary depending on the number of reflections of the light on the inner surface of the light guiding member. Specifically, light emitted from the light guiding member in the vicinity of the LED is reflected less frequently inside the light guiding member, whereas light emitted from the light guiding member at a location remote from the LED is reflected more frequently inside the light guiding member. Since the light reflected less frequently is diffused to a lesser extent by the reflection, the light is emitted within a narrow range in the sub-scanning direction in the vicinity of the LED. On the other hand, since the light reflected more frequently is diffused to a greater extent by the reflection, the light is diffused and emitted over a wide range in the sub-scanning direction at a location remote from the LED.
If, as described above, the distribution of light in the sub-scanning direction varies depending on the position in the main scanning direction, the level of change in illuminance caused by floating of a document varies depending on the position in the main scanning direction. This is inconvenient in that the degree of non-uniformity in illuminance caused by floating of a document may increase.
An illuminating unit is known in which the entire output surface on a light emitting side of the periphery of a light guiding member, the output surface having an area about half that of the periphery of the light guiding member, is provided with a diffusing member that diffuses light with a grained surface or a surface with rounded grooves and ridges. Light emitted from the output surface is diffused by the diffusing member in the sub-scanning direction. This can reduce the degree of non-uniformity in the distribution of light in the sub-scanning direction. Additionally, with this illuminating unit, where the diffusing member of the light guiding member is located in the vicinity of the LED, light having a narrow range of distribution can be effectively diffused.
However, in the illuminating unit described above, not all the light emitted from the light guiding member in the vicinity of the LED has a narrow range of light distribution in the sub-scanning direction. That is, the light emitted from the light guiding member in the vicinity of the LED includes light that has a wide range of light distribution in the sub-scanning direction. Therefore, of the light emitted from the light guiding member in the vicinity of the LED, light originally having a wide range of light distribution in the sub-scanning direction is also diffused by the diffusing member. A loss of light may occur when light is diffused by the diffusing member. Accordingly, if light originally having a wide range of light distribution in the sub-scanning direction is diffused by the diffusing member, there may be an unnecessary increase in the loss of light.